High density optical recording systems which may be used for recording and playing back information are known in the prior art. For example, U.S. Pat. No. 4,097,895, entitled "MULTI-LAYER OPTICAL RECORD" issued on June 27, 1978, in the name of F. W. Spong relates to an optical disc record/playback system wherein data is recorded in the form of pits in an absorptive coating on the surface of an optical disc. To put it in another way, the thermal energy of a focused high intensity light beam causes variations of the optical properties on the surface of the recording medium. In the Spong system, approximately 10.sup.11 bits of information can be stored on one side of a disc-shaped record medium having a 30 cm diameter.
In systems such as Spong it is necessary to control the bit sizes of the data being recorded. If the duty ratio of the pattern of information recorded on the record medium surface deviates from that of the signals being recorded the bit error rate of the system may exceed that which is permissible. For example, when the information being written is modulated at a 50% duty cycle the recorded portions of the record medium surface will be equal in length, over a small region of the disc surface, to the unrecorded portions. Variations between the recording signal and that which is recorded affect the output signal recovered from the record medium surface, thus adversely affecting the bit error rate of the information recorded.
These variations may be caused by several different effects. One reason is that the energy of the recording source is lost into the substrate of the record medium. It takes more energy to record on the outside portions of a disc than on the inside portions when recording at constant angular velocity. Another reason is that the record level necessary to effect recording may differ from one record medium to another depending upon the composition of the record medium or variations in the thickness of the medium. A further reason is that there may be variations in the record medium itself. On one record medium, for example, the thickness of the recording surface may vary from one portion thereof to another thus affecting the power level necessary to effect recording.